1. Field of the Invention
This invention relates generally to a system and method for reducing the frequency of fuel cell stack stand-by mode events as the fuel cell stack ages, if necessary, and, more particularly, to a system and method for reducing the frequency of fuel cell stack stand-by mode events as the fuel cell stack ages by determining the irreversible stack voltage loss, and from that assessing if the rate of voltage degradation is too high, where the voltage would be expected to decay below a predetermined limit before the rated end-of-life service of the stack.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between the two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
When the fuel cell system is in an idle mode, such as when the fuel cell vehicle is stopped at a stop light, where the fuel cell stack is not generating power to operate system devices, cathode air and hydrogen gas are generally still being provided to the fuel cell stack, and the stack is generating output power. Providing hydrogen gas to the fuel cell stack when it is in the idle mode is generally wasteful because operating the stack under this condition is not producing very much useful work. Thus, it is generally desirable to reduce stack output power and current draw during these idle conditions to improve system fuel efficiency.
For these and other fuel cell system operating conditions, it may be desirable to put the system in a stand-by mode where the system is consuming little or no power, the quantity of fuel being used is minimal and the system can quickly recover from the stand-by mode so as to increase system efficiency and reduce system degradation. U.S. patent application Ser. No. 12/723,261, titled Standby Mode for Optiminazation of Efficiency in Durability of a Fuel Cell Vehicle Application, filed Mar. 12, 2010, assigned to the assignee of this application and herein incorporated by reference, discloses one known process for putting a fuel cell system on a vehicle in a stand-by mode to conserve fuel of this type.
There are a number of mechanisms from the operation of a fuel cell system that cause permanent loss of stack performance, such as loss of catalyst activity, catalyst support corrosion and pinhole formation in the cell membranes. However, there are other mechanisms that can cause stack voltage losses that are substantially reversible, such as the cell membranes drying out, catalyst oxide formation, and build-up of contaminants on both the anode and cathode side of the stack. It is known in the art to remove the oxide formations and the build-up of contaminants, as well as to rehydrate the cell membranes, to recover losses in cell voltage in a fuel cell stack. U.S. patent application Ser. No. 12/580,912, titled Automated Procedure For Executing In-Situ Fuel Cell Stack Reconditioning, filed Oct. 16, 2009, assigned to the assignee of this application and herein incorporated by reference, discloses one such procedure for reconditioning a fuel cell stack to recover reversible voltage loss.
The membrane within a fuel cell needs to have sufficient water content so that the ionic resistance across the membrane is low enough to effectively conduct protons. Membrane humidification may come from the stack water by-product or external humidification. The flow of reactants through the flow channels of the stack has a drying effect on the cell membranes, most noticeably at an inlet of the reactant flow. However, the accumulation of water droplets within the flow channels could prevent reactants from flowing therethrough, and may cause the cell to fail because of low reactant gas flow, thus affecting stack stability. The accumulation of water in the reactant gas flow channels, as well as within the gas diffusion layer (GDL), is particularly troublesome at low stack output loads.
Wet operation of a fuel cell stack, that is, operation with a high amount of humidification, is desirable for system performance and contaminant removal. However, there are various reasons to operate a fuel cell stack with a lower humidification. For example, wet operation can lead to fuel cell stability problems due to water build up, and could also cause anode starvation resulting in carbon corrosion. In addition, wet operation can be problematic in freeze conditions due to liquid water freezing at various locations in the fuel cell stack.
To meet vehicle acceleration and grade ability requirements at predicted stack end-of-life (EOL) or end-of-service (EOS), the stack voltage must be maintained above a predetermined limit. Permanent stack voltage loss is predominantly related to loss of cathode electrode performance, which in turn is a function of voltage cycling characteristics. When the fuel cell system enters the stand-by mode if the stack voltage falls towards 0 volts, the subsequent increase in stack voltage after the fuel cell system exits the stand-by mode causes some irreversible voltage degradation as a result of catalyst activity loss.
Because of their operating profile, a small subset of vehicle drivers could cause accelerated fuel cell stack degradation rates, resulting in unacceptable performance at the end of the vehicle's target life. In contrast, it has been shown that the driving characteristics of most fuel cell vehicle operators are such that the number of times that their vehicle cycles through the stand-by modes is low enough that fuel cell degradation caused by the voltage cycling referred to above is not enough to cause unacceptable performance loss prior to the stack's target end of life. The high severity drivers would experience poorer fuel economy if they were not permitted to enter the stand-by mode when the vehicle idles. Optimizing both vehicle life and peak system efficiency, which includes enabling the stand-by mode, is not possible with current fuel cell systems.